Heat exchanger

ABSTRACT

A heat exchanger has been developed with a helical insert (9) permanently mounted in a housing (2). Between the windings in the insert (9) there is formed a helical channel (20) for one heat exchange medium. The insert (9) is designed with a channel (10) for the second heat exchange medium. The heat exchanger is designed with a central tube (13) which is axially movable and rotatable. The central tube (13) is designed with scraper elements for removal of deposits in the channel (20). In one embodiment the scraper element is formed by a helical insert (15) of the same design as the insert (9). The insert (15) is designed with a channel (16) for the second heat exchange medium. In a further embodiment the central tube (13) is designed with one or more scraper arms (23) which may be liquid-cooled. Deposits are often formed on the heat transfer surfaces. A cleaning cycle is performed by means of axial movement of the insert (15) which is mounted on the central tube (13) towards the permanently mounted insert (9), thus causing the heat transfer surfaces to touch each other. A rotating movement is then performed, e.g., a part of a turn, while the surfaces are close to each other or in contact with each other, thus causing the deposits on the two surfaces to be rubbed or scraped off and thereby cleaning the channel (20).

This application is the national phase of international applicationPCT/No. 95/00075 filed May 5, 1995 which designated the U.S.

FIELD OF THE INVENTION

The present invention concerns a heat exchanger designed as a housingwith one or more helical inserts with a through-flowing heating orcooling medium, and devices for keeping the heat transfer surfaces cleanduring operation.

BACKGROUND OF THE INVENTION

The heat exchanger should maintain a good heat transfer performance whena medium flows through it which has a strong tendency to deposit acoating on the channel walls. In the following description this mediumis called "the primary medium" or "the process medium". The primarymedium may be a product flow from a process in the form of a gas withsolid particles, flue gas with soot, or a liquid. On the other side ofthe heat transfer walls flows a second medium, called "the secondarymedium" or "the service medium", whose task is either to cool or heat upthe primary medium. The secondary medium may be a gas or a liquid.

The helical insert has internal channels through which the secondarymedium flows. The cross section of the insert may be in the form of oneor more rectangular tubes adjacent to one another or several round tubesadjacent to one another, and for the sake of simplicity is called "tubespool" in the following description.

At one end of the cylindrical housing there is an intake for the primarymedium, which flows through the windings in the insert or inserts to theoutlet at the other end. The secondary medium can be parallel flow orcounterflow according to what is most suitable for the process.

The invention comprises a heat exchanger which is equipped with acentral tube which extends along the centre axis of the housing. Thecentral tube is both axially movable and rotatable. On the central tubethere is mounted a device for removal of deposits on the walls of thechannel in which the primary medium is conveyed.

On the heat transfer surfaces of a heat exchanger particles will oftenbe precipitated which will adhere to the surfaces and be deposited as acoating which will reduce the heat transfer. The performance of the heatexchanger is highly dependent on its having clean surfaces. It has beenshown that even a thin layer of particles or a thin coating of depositswill substantially reduce the performance. If a thicker layer of coatingis formed it will also narrow the channel opening, thus increasing theflow resistance and thereby obstructing the through-flow of the medium.

The temperature of the primary medium is sometimes so high that thecoating hardens after a short time and it thus becomes necessary to keepthe cooling surfaces clean in an efficient manner without the additionof foreign matter which will pollute the product flow.

A common problem with heat exchangers is that it is a relativelycomplicated process to remove fouling. Many different designs ofcleaning equipment are known and many methods for internal and externalremoval of fouling on tubes, plates, shell and housing.

The usual method of cleaning heat exchangers is to wash both the tubesand the housing with a liquid to which may be added a solvent for thefouling concerned. Another method which is used is to dismantle theentire heat exchanger and clean the whole tube bundle and housingmechanically by means of washing and brushing. However, both of thesemethods require the heat exchanger to be disconnected from the process,which is normally both a costly and laborious procedure.

In WO 88/01362 there is disclosed a heat exchanger with a plurality ofhelical tube spools wherein the tube spools are composed of a pluralityof parallel tubes located beside one another. The tube spools with adistributing head at each end are mounted on to a longitudinal centraltube, thus enabling the entire tube bundle with the distributing headsto be withdrawn from the housing. The dismantling process is therebyfacilitated, thus reducing the cleaning time. However, the heatexchanger is not designed to be self-cleaning or with cleaningequipment.

In NO 45071 there is disclosed a rotating heat exchanger withpermanently installed scraper devices. The scraper devices are locatedin the channels in which the flue gas is conveyed and will scrape offsoot on the cooled surfaces. The scraper devices, however, cover theentire channel cross section, thus making it necessary to direct theflue gas on both sides of the devices.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a heat exchanger whichis either self-cleaning or without external cleaning equipment, thusenabling the heat exchanger to be cleaned during operation.

This object is achieved according to the invention with a heat exchangerwith a central tube with scraper elements and which is characterized bythe features presented in the patent claims.

In one embodiment the heat exchanger consists of two tube spools, one ofwhich is permanently mounted on to the housing and the other mounted onto a movable central tube. By moving the two tube spools axially intocontact with each other and thereafter screwing them along each other,they will scrape or rub the cooling surfaces clean of deposits. Themovable tube spool is a part of the heat exchanger, thus eliminating theneed for additional elements for removing deposits and this is one ofthe advantages of the invention.

In a further embodiment of the invention one of the helical tube spoolswhich are mounted on to the central tube is replaced by scraperelements. These are preferably in the form of arms which are movedtowards the permanently mounted tube spool and which scrape the cooledsurfaces clean of deposits. The scraper arms can be designedsubstantially narrower than the channel, in such a manner that they donot obstruct the through-flow of the primary medium. In addition twosurfaces of the scraper arms are always scraped clean of any deposits,thus ensuring that they do not increase in height, and this is a furtheradvantage of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in connection with drawings whichillustrate examples of embodiments of a heat exchanger, only theprinciples of the invention being illustrated.

FIG. 1 is a longitudinal section through a heat exchanger with apermanently mounted helical insert and a helical insert mounted on to amovable central tube.

FIG. 2 is a longitudinal section through a heat exchanger with apermanently mounted helical insert and with scraper elements in the formof arms mounted on to the movable central tube.

FIGS. 3A, 3B and 3C show alternate embodiments of the scraper device.

DETAILED DESCRIPTION OF THE INVENTION

In the figures the same parts have the same reference numbers.

In FIG. 1 the heat exchanger is indicated by 1. It consists of a housing2 which is constructed with an internal wall 3. The housing 2 can alsobe equipped with an external wall 4 so that a channel 5 is formed. Thechannel 5 has an inlet 6 and an outlet 7 for a medium. The secondarymedium can be passed through the channel 5, the inner wall 3 of thehousing 2, thus contributing to the heat exchange. The housing 2 can bedesigned with a flange 8, thus enabling it to be mounted on to theoutlet opening for processing equipment, e.g. a reaction chamber.

A helical insert in the form of a tube spool 9 is mounted on to theinternal wall 3. The tube spool 9 preferably has greater width, i.e.extension in the radial direction, than height, which is the extensionin the axial direction. The tube spool 9 can have a rectangular,trapezoidal or triangular cross section. The distance between eachwinding in the tube spool 9 can be compared to a screw pitch and thenumber of windings can be chosen according to the requirements for heattransfer, etc.

The tube spool 9 is usually constructed of plates and the walls are theheat transfer surfaces. In some cases there is a need for high pressurein the secondary medium, e.g. in the production of steam by utilizingwaste heat from a process. In this case the helical tube spool 9 can becomposed of several tubes located beside one another, or the tube spool9 can be reinforced by means of welded-on stays. The secondary medium ispassed through the channel 10 in the tube spool 9 which is designed withan inlet 11 and an outlet 12.

The heat exchanger is designed with a central tube 13 located along thecentre axis of the housing 2. The central tube 13 is axially movable androtatable. The central tube 13 is passed through the housing 2 and thelead-through is sealed with a packing box 14 in the conventional manner.

On to the central tube 13 there is mounted a helical insert in the formof a tube spool 15 which has the same distance between the windings asthe tube spool 9. The tube spool 15 can therefore be fed into thehousing between the permanently mounted helical tube spool 9.

The secondary medium is passed through a channel 16 in the tube spool15. The tube spool 15 can have a rectangular, trapezoidal or triangularcross section and may be composed of several tubes located beside oneanother. The central tube 13 is designed with an internal tube 17, thusforming channels which convey and distribute the secondary medium to andfrom the tube spool 15. The central tube 13 is designed with an inlet 18and an outlet 19 for the secondary medium.

Both the tube spools 9 and 15 and the housing 2 contribute to the heatexchange, the secondary medium being passed through the channels 10 and16 and through the channel 5 in the housing 2.

Between the tube spools 9 and 15 which are located at a certain distancefrom each other, there is formed a helical channel 20, and the primarymedium is passed through this channel. By installing several paralleltube spools 9 and 15 the primary flow will be divided up into severalparallel courses.

The primary medium passes from the inlet 21 through the helical channel20 which is formed by the walls of the two tube spools 9 and 15, theinner wall 3 of the housing 2 and the central tube 13 and on to theoutlet opening 22.

The width of the tube spools 9 and 15 is adapted in such a manner thatit extends between the central tube 13 and the inner wall 3 of thehousing 2 with a certain clearance.

The construction elements in the heat exchanger can be made of variousmaterials depending on the operating temperatures of the primary andsecondary media employed.

Moreover, the direction of flow of the primary medium and the secondarymedium can be chosen according to the existing requirement for heatexchange and thereby parallel flow or counterflow heat exchange can beachieved in the known manner.

FIG. 2 illustrates an embodiment wherein scraper arms are mounted on thecentral tube. In other respects the heat exchanger is designed as FIG. 1and the same parts have the same reference numbers.

The heat exchanger is designed with a helical insert in the form of atube spool 9. Between the windings in the tube spool 9 there is formed ahelical channel 20 and the primary medium is passed through this channelfrom the inlet 21 to the outlet 22. The secondary medium is passedthrough the channel 10 from the inlet 11 to the outlet 12.

On to the central tube 13 which is axially movable and rotatable thereare mounted scraper elements in the form of scraper arms. Two scraperarms 23 are preferably mounted per winding of the tube spool 9, and thescraper arms 23 are then located diametrically. The number of scraperarms 23 can be increased, thus correspondingly reducing the size of therequired angle of rotation.

The scraper arms 23 are preferably designed in a cylindrical shape withgreater length, i.e. extension in the radial direction, than diameter,which is extension in the axial direction. The length of the scraper armis adapted in such a manner that it extends from the central tube 13 tothe inner wall 3 of the housing 2 with a certain clearance. The scraperarm 23 will thereby clean the inner wall 3 of the housing 2. The scraperarms 23 are designed much narrower than the width of the channel 20,thus ensuring that the through-flow of the primary medium in the channel20 is not obstructed. The number of scraper arms 23 in the channel 20 isalso adapted to a minimum, thus ensuring that the through-flow of theprimary medium is obstructed to the least possible extent.

If necessary the central tube 13 and the scraper arms 23 are cooled. Inthis case the scraper arms are equipped with an internal tube 24, thusforming channels for a cooling medium. The tubes 24 are mounted on to aninternal tube 17 in the central tube 13. There are thereby formed in thecentral tube 13 channels which convey and distribute a cooling medium tothe scraper arms 23. The cooling medium, which can be the secondarymedium, is introduced through the inlet 18 and discharged through theoutlet 19 in the central tube 13.

The apparatus works in the following way and an example of a cleaningcycle is described. Other cycles may be used. The heat transfer surfaceswith deposits are cleaned by moving the central tube 13 with the tubespool 15 axially, e.g. in the direction towards the inlet 21, until thewalls of the tube spool 15 are in contact with the walls of the tubespool 9 or at a defined distance from each other or until the depositstouch each other. The cooling surfaces are preferably moved close toeach other but in such a manner that they do not come directly intocontact with each other. This prevents wear on the surfaces, which initself is a disadvantage. In addition it prevents materials which may bescraped off the heat transfer surfaces from polluting the primarymedium.

The central tube 13 is then rotated a half turn, e.g. in a clockwisedirection, while at the same time the walls of the tube spools 9 and 15are kept at the same distance from each other. The movable tube spool 15is thereby screwed along the permanent tube spool 9 and deposits arescraped or rubbed off the wall surfaces in the entire channel opening.

The next stage in the cleaning process consists in the central tube 13being moved axially in the direction towards the packing box 14 untilthe walls of the tube spools 9, 15 are in contact with each other. Thecentral tube 13 is then rotated a half turn in an anticlockwisedirection, thus causing deposits to be scraped or rubbed off thesurfaces.

Finally the central tube 13 is moved in such a manner that the tubespool 15 is placed in a neutral position.

In order to cover both sides of the ends of both inserts by causing theinserts to be rubbed against each other, they have to rotate at leastone turn in relation to each other. At a point where the surfaces covereach other, the rubbing movement, i.e. where the surfaces are screwedalong each other and touch each other, may be short in order for thedeposits to break off. If desirable the rotational movement can bereduced, but this will cause the cleaning effect to be reduced on a partof the end surfaces of the insert.

The cleaning cycle can be performed with the same steps when scraperarms 23 are mounted on the central tube 13. It may, however, benecessary to rotate the central tube 13 one or more turns each waydepending on the number of scraper arms 23 mounted on the central tube13.

By means of a cleaning cycle of this kind all cooled surfaces arescraped in the channel 20, both walls of the tube spools 9 and 15, theinner wall 3 of the housing 2 and the outer surface of the central tube13. This is one of the advantages of the invention.

In addition the tube spool 15 or a scraper arm 23 will clean thecylindrical inner wall 3 for some distance above the entrance to thehelical channel 20. The length of the cleaned surface can be selected bymeans of the design of the central tube 13 and the axial movementthereof. A scraper arm 23 may be mounted outside the tube spool 9.

At the outlet of a reactor, boiler or the like there normally occurssome narrowing of the flow cross section which in turn can cause a largeconcentration of particles or deposits. By placing the heat exchangerunder a reaction chamber or boiler room the tube spool 15 or one or morescraper arms 23 will have a lifting and rotating movement, thus causingloose materials above the heat exchanger to fall down and follow theproduct stream out of the system.

The cross section of the channel 20 is selected in order that the flowvelocity of the primary medium will be sufficient to enable the depositswhich have been scraped loose to follow the flow out of the heatexchanger. Moreover, by making the correct choice of scraping directionin relation to the force of gravity, the scraper arms 23 can help tofeed by stages deposits which have been scraped loose out of the heatexchanger.

The heat transfer surfaces in a heat exchanger preferably have a smoothsurface. As shown in FIGS. 3A, 3B and 3C, in order to increase thecleaning effect one surface or both the surfaces which come into contactwith each other during the cleaning stages can be equipped with brushes24', a rough or grainy surface, grooves or ridges 24'" with a certainpattern or with knives, scraping edges 24" or cutting edges. This is notillustrated in the drawings.

In one embodiment the surface can have an uneven shape, e.g. acorrugated shape. The deposits will then be exposed to varying loadswhen the surfaces are rubbed against each other and will be more easilybroken up.

In a further embodiment the surface can be equipped with grooves such asridges with a kind of pattern in which the grooves, e.g., are slantingin relation to the radial direction. When the surfaces rotate inrelation to each other the deposits will move sideways and be pushed outof the pattern.

The central tube 13 can be connected to a device which may bemotor-driven, e.g. hydraulically operated, the central tube thusperforming the axial to and fro movements and the rotational movementswhich are necessary for a cleaning cycle.

A cleaning cycle can run continuously or intermittently and the cleaningrate can be controlled, e.g., by the temperature difference betweeninlet and outlet for one of the media or by the outlet temperature forone of the media when the inlet temperature and the flow rate areconstant.

Temperature sensors 25, e.g. thermoelements, can be placed both at theinlet opening 21 and the outlet opening 22. A drop in the temperaturedifference for the primary medium between the two measurement pointswill indicate that the heat transfer is being reduced due to theformation of deposits and this can start a cleaning cycle or increasethe rate thereof.

With a heat exchanger according to the invention the cleaning can beperformed during operation. It is not necessary to stop a process eitherin order to wash a heat exchanger or in order to dismantle it forcleaning.

It is claimed:
 1. A heat exchanger comprising a housing having a centralaxis and a permanently mounted helical insert which forms a channel forone heat exchange medium, said insert including at least one channel fora second heat exchange medium, a central tube placed along the centralaxis of said housing and including a scraper device, said central tubeand said scraper device being axially movable and rotatable in andrelative to said housing, said scraper device comprising a helicalinsert of the same type as said permanently mounted helical insert andincludes a channel provided therein in flow connection with the secondheat exchange medium via said central tube.
 2. A heat exchangeraccording to claim 1, characterized in that there are provided aplurality of scraper arms symmetrically around the central tube in eachwinding in the permanently mounted helical insert.
 3. A heat exchangeras claimed in claim 1 wherein said scraper device has at least onesurface permanently mounted in said housing with said surface providedwith scraping members.
 4. A heat exchanger as claimed in claim 3 whereinsaid scraping members include brushes.
 5. A heat exchanger as claimed inclaim 3 wherein said scraping members include cutting edges attached tosaid surfaces.
 6. A heat exchanger as claimed in claim 3 wherein saidscraping members include grooves on said surfaces.
 7. A heat exchangercomprising a housing having a central axis and a permanently mountedhelical insert which forms a channel for one heat exchange medium, saidinsert including at least one channel for a second heat exchange medium,a central tube placed along the central axis of said housing andincluding a scraper device, said central tube and said scraper devicebeing axially movable and rotatable in and relative to said housing,said scraper device including at least one scraper arm which is tubularin shape having a greater length than diameter, said scraper arm forminga channel in flow communication with the second heat exchange medium.